So, even though I've been doing alterations for many, many years, I still make my share of mistakes, most of which are easy to correct. Don't let the possibility of messing something up stop you from trying. Experiment, practice and keep moving on!
Obviously, this zipper replacement did not turn out well. When I got it all finished, the two sides were off by a good ½".
After closer examination, I can see that the zipper teeth do not even line up correctly.
And I can see that the problem began at the very start, when I did not line up the two sides of the slacks at the base of the zipper.
Here you can see that the fabric has some stretch in it. Knowing that the ends were not matching up well, I tried to stretch one side to fit into the other, thinking that a little wrinkling would come out with a good burst of steam. But, oh, not so. You can't fool the fabric into behaving when it wasn't set right in the beginning. And what is up with that shotty looking bar tack to the left of the zipper? Was I in a hurry?
Ahhhh, that's much better. Take out the one side of the zipper, realign and put it back in. Then since I had to remove that bar tack anyway, I resewed it, making it a much more professional looking job.
In the end, there is no substitute for attention to detail. I tried to rush this job through and ended up taking additional time, because I had to correct my mistakes. On this lesson, I didn't learn how to put in a zipper, I learned how to use patience to put in a zipper well.